1. Field of the Invention
The present invention relates to polymer-based optical fibers possessing non-linear optical activity. The organic or inorganic optical fiber prepared by the method described herewith exhibits electro-optic activity and frequency-doubling capability in addition to serving standard light transfer and data transmission functions.
2. Description of the Technical Background of the Invention
The advent of semiconductor GaAs lasers with a fundamental wavelength between 0.8 and 1.5 micrometers represents a major technological breakthrough. However, the long wavelength of such sources has limited the utility of semiconductor lasers. A non-linear optical material that generates higher harmonics from monochromatic inputs with long wavelengths will find a variety of practical applications, such as in electrophotography, optical switching and scanning, mass data storage, and integrated optoelectronics, among others. These applications have stimulated considerable efforts toward the understanding and manufacture of non-linear optical material. Polymers enter into the field as hosts for dispersed small molecules that possess non-linear optical properties.
The fundamental concepts of non-linear optics can be easily understood in mathematical terms. In the dipolar approximation, the polarization P induced in a chemical structure (atom or molecule) by an external field E may be represented by the following equation: EQU P=.alpha.E+.beta.EE+.gamma.EEE+. . . (1)
where the vector quantities P and E are related by the tensorial quantities, .alpha., .beta., and .gamma. (referred to as the polarizability, hyperpolarizability, and second hyperpolarizability, . . . etc.) These latter quantities depend on the detailed architecture and electronic dispersion of the various parts of the chemical species under study, and are microscopic (molecular) properties. When individual molecules aggregate to form macroscopic samples, the polarization induced by an external field in the bulk media may be expressed by the following equation as: EQU P=.chi..sup.(1) E+.chi..sup.(2) EE+.chi..sup.(3) EEE+. . . (2)
where the microscopic properties .alpha., .beta., and .gamma. are replaced by the corresponding macroscopic quantities .chi..sup.(1), .chi..sup.(2), .chi..sup.(3) . . . (the first, second, third . . . harmonic coefficients).
Organic molecules that are conjugated and polarized, i.e., containing electron donor and acceptor pairs oppositely positioned, exhibit appreciable molecular hyperpolarizability, .beta..sup.(1). A simple example is p-nitroaniline, which possesses strong electron-withdrawing and donating groups spaced by an aromatic ring. When an incoming electromagnetic wave interacts with such a noncentrosymmetric entity, the wave form is distorted (rectified) in such a manner that the output comprises significant components of higher harmonics.
The mere existence of molecular noncentrosymmetry does not guarantee that macroscopic samples exhibit nonzero non-linear optical coefficients, e.g., .chi..sup.(2). The spatial arrangement of the molecules must meet the condition that macroscopically the sample remains noncentrosymmetric as well. Equivalently, the dipole moments of the molecules must have a preferred alignment. Unfortunately, due to energetic considerations, most substances condense into a configuration where the dipole moments are antiparallel. Hence, molecular non-linearity is cancelled in bulk samples.
It was not until the first "poling" experiments were performed that the situation began to improve. Poling is, in essence, molecular alignment induced by a high imposed direct current (dc) electric field. This externally applied field imparts a preferred orientation to the dopants in a polymer matrix at high temperatures. Quenching freezes in place, this externally induced alignment, as the high viscosity of the solidfied, amorphous matrix preserves the orientation. However, only electro-active an frequency-doubling thin films have been prepared in this fashion. In addition, the effectiveness of dipole alignment by the poling process is limited. The specific system investigated in the poling experiment in the literature used an azo dye (Disperse Red 1, 4-[N-ethyl-N-(2-hydroxyethyl)] amino-4'-nitroazobenzene) as the non-linear optic dopant, and PMMA as the amorphous host polymer. Aside from the observed second harmonic generation, such polymeric host-guest systems possess good film-forming properties, which may qualify them for integrated optics application.
In the present invention, a method and device for the preparation of optical fibers having distinct non-linear optical properties is disclosed. The preparation technique involves a specially designed fiber spinner which exploits the combined electric and/or magnetic-field-induced and elongation-flow-induced orientation of doped noncentrosymmetric molecules (or chemically attached noncentrosymmetric moieties). This combined electromagnetic/flow field approach results in efficient rod alignment with a high degree of directional anisotropy. The end products are optical fibers with the rigid-rod-like noncentrosymmetric species aligned parallel to the fiber axis while retaining a preferred dipole orientational anisotropy. Such optical fibers are expected to become a significant force in the emerging field of opto-electronics.